No matter which sport you are going to see when the third London Olympics begin, a visit to 2012 park will mean one thing – walking though a very large shopping centre first.

The high-speed Javelin train from King's Cross – set to deliver 25,000 spectators an hour to Stratford International – exits to a busy row of shops and restaurants, constructed by the Westfield Group.

Crowds arriving at Stratford's tube and mainline station can exit either via a concourse leading directly into Westfield's complex or walk across an elegant rusted steel bridge – again built by Westfield, again delivering sports fans into the heart of the retail development.

In all, for 70% of visitors, the entrance to the Olympics will be through the vast shopping development. Welcome to London 2012? Welcome to Westfield first. There are now fewer than four weeks until the 13 September opening of what claims to be the biggest "in town" shopping centre in Europe.

Thousands of workers in fluroescent vests and hard hats race to and fro across the Westfield bridge to hang panelling, finish the electrics and install street furniture on the 180-acre site.

Behind closed doors or plywood screens, the 300 shops, 50 restaurants, three hotels, 17 cinema screens and casino are being fitted out. The complex is already heavily branded, with the red Westfield logo stamped on buildings at almost every vista.

Westfield Stratford City – as the vast retail and leisure complex has immodestly been named– is far from beautiful but there is no disputing that this enormous retail sprawl is remarkable.

Most striking, perhaps, is the degree to which it and the Olympic park development, nearing completion just to the east, are entangled.

The history of Westfield's east London outpost ("Westfield East" and "Eastfield" were never considered as names, despite reports to the contrary, insists the company) began in 2004 when it acquired the struggling developer Chelsfield, and with it plans for an enormous retail and leisure complex in Shepherd's Bush, west London, and a 25% stake in an even bigger project earmarked for east London. There was no question which was the more attractive prize.

The brownfield Stratford site, unlike the Shepherd's Bush project which has become a lucrative flagship for the Australian group, required vast investment in infrastructure. At that stage, says John Burton, the development director of Westfield Stratford City, no one expected London to win the Olympics. "So while we knew there was an opportunity here, what we couldn't get our minds around was when that might occur."

Hosting the Olympics focused minds. "I suppose, if it hadn't been that our chairman and MD said that we would deliver this in time for the Olympics, we probably would have delayed it."

The developer gave the Olympic Delivery Authority a significant leg-up, having already secured planning permission for 5,000 homes – most of which will serve as the athletes' village before being resold – and investing heavily in transport infrastructure. One might consider unprecedented free global publicity, and a vast, captive and carefully shepherded audience for a fortnight, a rather nice thank you.

Of course, aside from the "city" of the developer's fantasies (a sense underscored by the fact that its postcode will be E20, until now existing only in BBC1's EastEnders), there is a real Stratford.

Cross Great Eastern Road from Stratford station rather than bearing left over Westfield's bridge, and there is palpable apprehension among locals who have watched the complex rise from the ground.

The suburb already has a shopping complex, the Stratford centre; if it was ever glossy, it isn't now, with its low ceiling and over-bright strip lighting and tired lino floor. There is a 99p shop, and a cheap fashion shoe store, and traders selling plastic flowers and clothes on rails marked £3, £4, £5.

Through the arcade, on High Street, is a row of market traders under permanent steel gazebos. They are being moved, they are not sure when, to the top end of the street, even further from any Olympic crowds or, post Games, Westfield shoppers who might lose their way and stumble into real east London.

"It's probably going to kill us," says Mike Wischnia who is minding his girlfriend's clothes stall. She has traded here for a decade, but business is horribly slow and he fears they'll have to pack in. "Everything is going to be over the other side," he says. "What's the incentive to come over here?"

Other traders, like many shoppers, are prepared to be open-minded for now. "It could be good, Westfield," says one stallholder, selling nail varnish and hair accessories. "We haven't got any 'name' shops any more."

Mostly, however, they feel uncertain and a bit apprehensive, she says.

"The traders want to know what's happening, I get that," says Sir Robin Wales, the ebullient and refreshingly frank mayor of Newham, who has been cheerleader for the retail and sporting developments.

"The centre and the market stalls are a value offer, which is really important for people who live in Newham, and we'll defend that. I don't want gentrification that drives people out. I want jobs for our people, and I want them to be able to shop in places where they can afford to. They may or may not shop at Westfield. I don't care. My issue is to make sure they are able to work."

"Jobs, jobs and jobs" has been Wales's mantra since the earliest days of the development. On this, he says, Westfield and many of its client retailers, including John Lewis, have been "absolutely stunning".

It has been his aim to ensure that 2,500 of the 10,000-plus jobs at the development go to local, long-term unemployed. The developer agreed to a "retail academy" to train locals.

It's all part of a longer-term, and vastly more ambitious, plan of convergence says Wales, where the six host boroughs want to raise east London, historically and intransigently poor, to average levels for the capital in jobs , poverty and health.

About 20,000 long-term unemployed people will need to get into work for Newham to come close. "A century of deprivation. This is us trying to tackle it."

The challenge for the complex – and the city-changing powers it claims for itself - will come on 10 September next year, almost a year after its opening, when the Paralympics are over, and the crowds have peeled away, and Westfield Stratford City becomes just another shopping centre.